The invention relates to a zigzag classifier with a classifying space and the associated air supply equipment.
A zigzag classifier for vertical tube air classifying of particulate products is known from the U.S. Pat. No. 1,861,248. This classifier has a plain, vertical, tube which is smooth on the inside and has a rectangular cross-section and which is inclined alternately to the right and to the left at the same angle to the vertical. The coarse fractions of the feed material slide downwards on whichever wall of the duct is underneath at the time. At the bends they have to cross through the classifying air flow coming from below. During each crossing a separation of fines takes place which in itself does not lead to a sharp classification. However, by multiple repetition a very sharp separation of the fractions can be achieved ultimately due to a multiplying effect.
The throughout of such a classifier increases with its cross-sectional area which, however, cannot be increased at will for reasons of uniform through-flow of classifying air, of fairly uniform distribution of material, of the thickness of the material layer and the like. However, if still more bends are provided to even out such irregularities, the dimensioning of the classifier and the control of the correct dimensions becomes problematical. This is particularly disadvantageous if the classifier is to be combined with other equipment such as, for example, with a fluidized bed spray granulator (see, for example, EP No. 163 836) to form a process engineering unit.
A pneumatic classifier, which is, for example, described in DE No. 1 507 686, was also developed for dedusting particulate material at high throughputs. In this classifier a strongly-fluidized fluidized bed is combined with a system of parallel zigzag ducts arranged above it. In this instance the feed bed travels through the fluidized bed in such a way that at the end coarse material emerges from the bed. The fine-material is checked for particle size in the zigzag ducts and, if it falls below a predetermined limiting particle, it is transferred by the classifying air into a collector. The coarse particles, on the other hand, fall back into the bed.
In addition, there are multi-duct zigzag classifiers on the market in which the material to be classified is supplied uniformly to the ducts by a distribution screw. The classifying air flows to the parallel ducts without metering. In order to avoid instability such classifiers should only be operated at a low solid loading.